In crystalline solar cells, hitherto, attempts to improve energy conversion efficiency by using so-called “optical confinement effect” through converting a flat surface of a silicon substrate to a surface concaved or convexed are made. In this approach, since light reflected once on an inclined surface of the concave and convex is received and taken at the adjacent inclined surface of the concave and convex as distinct from a substrate with a flat surface, the reflectance at the surface can be substantially reduced. As a result of this, since overall quantity of incident light increases, conversion efficiency is increased.
As a method of forming the above-mentioned concave and convex structure, for example, there is proposed a method of immersing a silicon substrate into a catalytic metal ion-containing mixed aqueous solution of an oxidizing agent and hydrofluoric acid (Patent Document 1). It is disclosed that in accordance with this method, a porous silicon layer can be formed on the surface of the substrate.